The present invention relates to an optical display system that may be used with head mounted or hand held display systems. More particularly, the invention relates to a two-stage optical system for this display system that comprises a first stage for magnification and image sizing and a second stage that includes a total internal reflection process.
A real image refers to an image that is observed directly by the unaided human eye. A photograph is an example of a real image. Electronic displays that provide a real image generally provide some form of display surface on which the real image is formed and viewed. A real image can be observed by the unaided eye when a viewing surface is positioned at its location. Examples of electronic displays that provide real images include liquid crystal displays, CRT monitors, and projection screens.
In contrast with real images, a virtual image is an image that appears to be coming from a location where no real image exists. By definition, a virtual image can exist at a location where no display surface exists. The size of the virtual image therefore is not limited by the size of a display surface. An example of a virtual image is the image of fine print viewed through a magnifying glass. The print not only appears larger, but it also appears to be located substantially behind the surface where the print actually exists. Virtual image displays thus have the advantage of eliminating the need for a large display surface in order to provide a large image to the viewer.
Software and computer hardware for creating virtual images have improved steadily over time. However, generating sizable displays is expensive and greatly increases the cost of display devices. In order to provide a viewer with as complete a virtual reality experience as possible, the image he sees should fill his field of vision. The viewer must also be able to look around at his environment. In order to accomplish these goals, displays need to provide a virtual image to the viewer as opposed to a real image.
A virtual display must initially form a source image that is then rendered by an optical system to create the virtual image. A substantial advantage of a virtual electronic display is that the source image initially created may be as small as can be usefully reimaged by the optical system. As a result, virtual electronic display systems may effectively use very small displays to form the source image. Pixel sizes may be as small as a few microns in diameter. At this size, the unaided eye cannot resolve images. Rather, in order to view the source image formed by the display, substantial magnification of the optical system is required.
A virtual image must be created by an optical system of some kind. In a real image electronic display, it is the eye and the viewing surface properties that determine the viewing parameters. By contrast, in a virtual image display, the optical system determines most of the viewing parameters.
There are three important parameters relating to the ease of viewing the image associated with virtual image display systems. The first parameter is the eye relief. This refers to the maximum distance from the eye which the optical system can be held and have the eye still see the entire virtual image. Optical devices which provide a eye relief which is a short distance from the optic are undesirable due to the inconvenience and discomfort associated with placing the eye in close proximity with the optic. It is therefore preferred that an optic provide a long eye relief in order to enable the magnified image to be viewed through the optic at a comfortable and convenient range of distances from the optic.
The second parameter relating to the ease of viewing a virtual image is the apparent angular width of the virtual image, commonly referred to as the field of view of the virtual image. The full field of view is defined as the ratio of the largest apparent dimension of the virtual image to the apparent distance to the virtual image. It is generally equivalent to the field of view for a real image display surface.
The third parameter relating to the ease of viewing a virtual image is the transverse distance that the eye may move with respect to the optical system and still have the eye see the entire virtual image through the optical system. This is commonly referred to as the xe2x80x9ceyebox.xe2x80x9d The size of the eyebox is determined by the eye relief and size of the exit pupil of the display system. The exit pupil is the place where the eye must be placed in order to see the whole image at once. A large exit pupil has been found to be one of the most important factors in determining viewing comfort by nearly all users we have tested. A large exit pupil and eyebox will also accommodate the range of pupil motion needed as the eyeballs rotate to scan through the viewing angle of the image, as well as to accommodate the variation in the interpupillary distance among the user population.
A need currently exists for an inexpensive, compact, virtual image display system that can be used in a hand held or head mounted apparatus that provides a virtual image that is positionable within a small volume and has a large exit pupil and eye relief and uses small displays.
It is recognized that one of the primary factors driving up the cost of virtual reality display systems is the cost of the initial display. Prior art display systems have been created that use small displays coupled to magnification systems. These generate the larger virtual images the viewer sees. However, prior art magnification processes are bulky and can make head mounted or handheld display systems unwieldy and cumbersome. Also, these display systems do not work as well with handheld devices.
Therefore, the need exists for a lightweight display system that operably presents a visual display occupying all or almost all of the viewer""s field of vision to a wearer that is both comfortable and relatively inexpensive.
Further, video images are recorded with various aspect ratios, e.g., 4:3 and 16:9. The display screen showing the unmagnified video image will necessarily have fixed dimensions. Therefore, for all images with aspect ratios that do not match the fixed dimensions of the display screen, there will be distortion in either the height or the width of the image dependent upon the relation between the aspect ratio of the image and that of the display. It would be an improvement if the image could be adjusted so that it appears undistorted to a viewer.
An object of the invention is to create a display system that is lightweight, convenient, and relatively inexpensive.
This and other objects of the invention are accomplished by a display system having a two-stage optical system where the second stage magnification is accomplished using total internal reflection techniques. This two-stage system is usable in relatively compact and inexpensive display systems. In a preferred embodiment, the head-mounted display system has two sections extending rearward around the sides of the head. Within each section, a display screen projects an image that passes through a first lens that adjusts the size of the image. It can magnify the image, alter the aspect ratio of the image, or both. The image then undergoes total internal reflection within a subsequent lens, resulting in magnification of the image. The viewer is presented with a virtual image many times larger than the original display.